A reed switch comprises confronting contact points including magnetic wire members and a glass tube enclosing those points and performs open/close operations of the contact points by applying a magnetic field from the outside of the glass tube. The encasement of the magnetic wire members into the glass tube is carried out by heating, softening, and sealing both ends of the glass tube in an inert gas, in a reducing gas, or in a vacuum in the state where the magnetic wire members are inserted in the glass tube. In the heating operation, use is made of a heat source available under the above-mentioned atmosphere, for example, an infra-red radiation type heat source using a halogen lamp with a reflecting plate for light convergence.
Under the above-mentioned circumstances, an infra-red absorption glass specially developed is used as a glass for a reed switch. Presently, use is widely made of the infra-red absorption glass having a characteristic such that an infra-red transmittance at a wavelength of 1050 nm is on the order of 15% to 20% when the glass has a thickness of 0.5 mm.
In the meanwhile, the reduction in size of electronic components is strongly desired as an essential condition in order to achieve the reduction in size and weight of electronic apparatuses which is remarkably accelerated in recent years. For the reed switch, reduction in size similarly proceeds. Accordingly, the reduction in diameter, length, and thickness of the glass tube which determines the size of the reed switch is carried out.
However, as the reduction in diameter, length, and thickness of the glass tube for a reed switch proceeds further, the following problems are caused in a conventional infra-red absorption glass to decrease the process yield and to place a limit in further reduction in size.
A first problem is as follows. Specifically, an infra-red ray from the halogen lamp forms a spot having the spread on order of 10 mm in diameter even if converged. When the center of the infra-red spot is brought into coincidence with the center of a sealing portion of the reed switch using the glass tube having a short length and a small thickness, a peripheral portion of the spot hits and heats the contact points of the switch which should not be heated in its nature. Therefore, it is necessary to intentionally deviate the center of the spot from the center of the sealing portion and to heat the sealing portion by the peripheral portion of the spot. However, the peripheral portion of the spot is low in infra-red energy and is unstable. Accordingly, the sealing requires a long time so that the productivity is decreased. In addition, nonuniformity in sealed pattern is increased so that the yield is deteriorated.
Next, a second problem is as follows. Upon sealing, the glass is in a softened state at about 1000° C. and a glass component is evaporated although in a very small amount. Such evaporated component is again solidified on a surface of metal material and/or of the glass of the reed switch which are present in the vicinity and still relatively low in temperature. In case of the reed switch having a short length, the evaporated glass component is again solidified around the contact points of the switch to thereby cause contact fault (electrical connection defect) of the switch.
In view of the above-mentioned problems, this invention has been made. It is an object of this invention to provide an infra-red absorption glass suitable for efficiently producing a small-sized reed switch.